The activity of suppressor cells in regulating immune responses is now widely accepted, but the ability to control this regulation awaits a better understanding of the induction as well as the action of these cells. Our previous work has defined a feedback suppression system in which suppressor T cells are activated by immune 'inducer' cells. The cells capable of inducing normal cells to act as suppressor cells will now be specifically identified and characterized and the conditions affecting their appearace and cellular interactions defined. Immunized cells produced in different ways will be tested for the existence of two 'inducer' cell subpopulations (or more) with feedback suppression inducing activities. This study of the 'inducer' cells will include the kinetics and mode of their generation as well as the cells required, the types of antibody responses in which they function, and the ontogeny of their function. The importance of these cells in regulating the antibody responses of adult and developing animals will be considered as well as their possible involvement in the induction of tolerance, specifically self-tolerance, in the neonatal state.